1. Field of the Invention
The present invention relates to a positive resist composition for use in a manufacturing process of semiconductor devices, such as IC, manufacture of circuit substrates for liquid crystals, thermal heads and the like, and lithographic processes of other photo-fabrication, and also relates to a pattern-forming method using the same. In particular, the invention relates to a positive resist composition suitable for exposure by means of an immersion projection exposure apparatus with far ultraviolet rays of the wavelengths of 300 nm or less as exposure light sources, and a pattern-forming method using the same.
2. Description of the Related Art
With the progress of fining of semiconductor elements, shortening of the wavelengths of exposure light source and increment of the numerical aperture (high NA) of projection lens have advanced, and now exposure apparatus of NA 0.92 using an ArF excimer laser having the wavelength of 193 nm as the light source have been developed. As shown by the generally well known following expressions, resolution and the depth of focus can be expressed by the wavelength of exposure light source and the numerical aperture of the projection lens:Resolution=k1·(λ/NA)Depth of focus=±k2·λ/NA2 wherein λ is the wavelength of the exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are the coefficients concerning the process.
For further higher resolution by the shortening of wavelengths, an exposure apparatus with an F2 excimer laser having the wavelength of 157 nm as the light source has been studied, but the materials of lenses for use in the exposure apparatus for the shortening of wavelengths and the materials of resists are extremely restricted, so that the realizations of the reasonable manufacturing costs of the apparatus and materials and quality stabilization are very difficult, as a result, there are possibilities of missing exposure apparatus and resists having sufficient performances and stabilities within a required period of time.
As a technique for increasing resolution in optical microscopes, a so-called immersion method of filling a liquid of high refractive index (hereinafter also referred to as “an immersion liquid”) between an objective lens and a sample has been conventionally known.
As “the effect of immersion”, the above resolution and depth of focus can be expressed by the following expressions in the case of immersion, with λ0 as the wavelength of the exposure light in the air, n as the refractive index of immersion liquid to the air, and NA0=sin θ with θ as convergence half angle of the ray of light:Resolution=k1·(λ0/n)/NA0 Depth of focus=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equivalent to the case of using exposure wavelength of the wavelength of 1/n. In other words, in the case of the projection optical system of the same NA, the depth of focus can be made n magnifications by immersion. This is effective for every pattern form, further, it is possible to combine an immersion method with super resolution techniques such as a phase shift method and a transformation lighting method now under discussion.
When an ArF excimer laser is used as the light source, it is thought that pure water (having a refractive index of 1.44 at 193 nm) is most promising as the immersion liquid in view of the safety in handling, the transmittance and the refractive index at 193 nm. When an F2 excimer laser is used as the light source, a solution containing fluorine is discussed from the balance of the transmittance and the refractive index at 157 nm, but a sufficiently satisfactory solution has not been found yet from the viewpoint of the environmental safety and at the point of refractive index. From the extent of the effect of immersion and the degree of completion of resist, it is thought that immersion exposure technique will be mounted on an ArF exposure apparatus earliest.
As described above, it is expected that the application of an immersion exposure method will bring considerable advantages to the semiconductor industry in view of economy and lithographic characteristics, such as resolution and the like. On the other hand, since a resist film is directly brought into contact with an immersion liquid at the time of exposure in immersion exposure, it is feared that, in the case of ArF immersion, the components of the resist film are eluted into pure water of the immersion liquid to soil the lens of exposure apparatus, or pure water osmoses into the resist film and exerts a bad influence on resist performance.
There is disclosed in WO 2004/068242A1 a resist composition that does not impair the improving effect of resolution and the depth of focus that is the advantage of immersion lithography, and influenced by water used in an immersion lithography process with difficulty.
However, further improvement of conventional resist compositions is required in the points of elution of the components of resist films into pure water of the immersion liquid and development defect.